Reinforced composite structural panels, typically sheet laminate structures, are well-known and frequently used in a variety of structures where other structural materials, such as metals, are not desired. These reinforced composite sheet laminate structures are used in the production of vehicles, such as boats, in some automobile bodies, and in aircraft. The reinforced composite materials are highly effective in their strength to weight ratio and thereby afford significant advantages over the conventionally used structural materials of wood, metals and the like.
Typically, these reinforced composite panels are formed of glass fiber-synthetic resin materials. Moreover, they are often constructed as multi-ply panels or so-called "sandwiched panels."
The use of multi-layer panels with foam cores has also been known. These foam cores are also provided with surface layers of resin-impregnated glass fibers. However, the presently available reinforced composite panels which utilize foam cores are subject to several limitations, most notably that when used under high load conditions, they frequently delaminate.
One such reinforced panel which utilizes a glass fiber skin and a foam core is taught in U.S. Pat. No. 4,411,939 to Hawkins, et al. The panel in the Hawkins, et al. patent is capable of being conformed to a desired shape, as for example, being curved to shape the hull of a boat. The skins of the laminated structure in the Hawkins, et al. patent utilize a polyurethane foam core along with glass fiber skins impregnated with a polyester resin. Interlocking strips of foam core are individually wrapped with a sheet of fiberglass and the strips are fit together and held in interlocking relation by stitching, staples, or adhesive. The stitches are used only for purposes of holding the fiberglass webs in place on the foam strips for later processing.
U.S. Pat. No. 2,692,219 to Slayter, et al. discloses the use of structural panels having an interior core with cement-like skins. The panel in Slayter, et al. is used primarily for heat and sound insulation partitions and the like. The panel has a core formed of a porous board into which glass fibers are bonded with a resinous material. The outer faces of the board are covered by layers of a cementitious material which is applied as a slurry and the panel is then heated to cure the cement. Prior to curing, however, glass fiber thread is stitched through the panel in a low stitch-density matrix, such as on 2-inch centers, with the looped ends of the thread extending from the faces of the porous board into the cement layers. Upon hardening of the cement, the loops are imbedded therein. The stitching is used to prevent delamination of the panel, but the panel would not have significantly improved out-of-plane (90.degree. to the surface of the panel) tensile strength. Thus, Slayter does not disclose a damage-tolerant structure.
U.S. Pat. No. 4,256,790 to Lackman, et al. discloses a composite structure in which composite panels (i.e., resin-impregnated fiber sheets) are bonded together at a joint which is then reinforced by a composite thread sewn in a series of stitches through the panels. Stitching is concentrated only in the most critical areas of the stiffened panel construction, namely, at the tension point of the radius between the skin and a stiffener and between opposite skins that make the web of a stiffener. Stitching also occurs at the point of the honeycomb structure where the opposite skins are in facewise contact with one another. The composite members are staged (i.e., heated and compressed to remove excess resin) prior to stitching.
U.S. Pat. No. 4,331,723 to Hamm discloses a structure for a joint in which a wedge-shaped composite insert is covered on its three sides by graphite-epoxy laminate layers. Adjacent the insert, the laminates are stitched together with a thread preferably of Kevlar.
U.S. Pat. No. 4,828,206 to Bruno, et al. discloses a wing structure for a hydrodynamic ram. This wing structure is in the form of a panel using glass or graphite skins which control the hydraulic ram effect of an exploding shelf inside the wing structure. Pairs of rows of stitching are used with one of the rows of a pair on each side of a bolted joint. The joints also have a ply build-up and the stitching is used to control the spread of damage and serve as a locating line for a "blow-out." The stitching is performed in a B-Stage lay-up, that is, in a wet stage which is stitched and then cured. This stitch line creates a line of weakness for controlling the location of a failure in a tension or compression loaded skin.
Finally, U.S. Pat. No. 5,308,228 to Benoit et al. discloses a gas turbine blade having a central core of pre-impregnated fiber material, the outer surfaces of which are covered by woven composite materials, preferably pre-impregnated fiber material. The entire construction is then held together by a thread stitched through the various layers. Density of stitching varies over the surface of the blade, being highest toward the tip and lowest toward the root. After stitching, the structure is then impregnated and polymerized in a mold to obtain a finished blade. Benoit discloses that the central core may include inserts of a non-resinous nature, such as metal or foam. The outer layers may be stitched prior to impregnation. Benoit is not concerned with producing a structure having improved tolerance for impact damage caused by a foreign object.
There has been a need for a sandwich panel with improved flatwise tensile strength and capable of substantially reducing damage propagation from impact by a foreign object, such as a falling tool box, a rock from the runway, etc. The panels currently in existence do not fulfill this need.